This invention relates to optical fiber-based digital delay lines. More specifically, it relates to a multi-fiber digital delay line incorporating nested optical fibers affixed to a substrate in a single physical plane.
Current broadband telecommunications networks are being configured to carry increasing volumes of voice, data and multimedia information. To meet these increasing volume demands, such networks are being implemented using optical communications systems technology. Prominent in present high-volume optical communications systems is the use of wave division multiplexed (WDM) or dense wave division multiplexed (DWDM) optical communications schemes that are capable of placing many optical channels centered at different wavelengths on a single fiber. WDM and DWDM networks require sophisticated optical switching capabilities in order to selectively route these many channels among a number of traffic-carrying fibers. In order to appropriately buffer and sequence individual channels to be routed, for example, such switches commonly employ optical delay lines.
Optical delay lines may be formed using a variety of optical technologies (see, e.g., Kenneth P. Jackson et al., xe2x80x9cOptical Fiber Delay-Line Signal Processing,xe2x80x9d IEEE Transactions On Microwave Theory And Techniques, Vol. MTT-33, No. 3, March 1985, pp 193, 194). Optical fiber delay lines have proven to be attractive due to their relatively low loss and low dispersion characteristics. Typical optical fiber configurations include recirculating delay lines, multi-tap delay lines and multi-fiber delay lines. A recirculating delay line incorporates a fiber which partially closes upon itself (for example, by means of an optical coupler) so that signals introduced at one end of the delay line recirculate around the loop to be output with each transit cycle. Non-recirculating optical fiber delay lines such as multi-tap and multi-fiber delay lines produce only a single output signal at each output port in response to each input signal.
The multi-tap structure consists of a fiber with taps distributed along its length, each tap being capable of providing a signal output. The output signals from each tap may be collected and output by an optical combiner. Relative delays among the output signals are controlled by the relative placement of taps along the fiber.
In a typical multi-fiber optical delay line, an optical signal is split and provided as input to two or more optical fibers of varying length (see, e.g., U.S. Pat. No. 5,703,708, issuing to Das et al. on Dec. 30, 1997). The optical signals on each fiber""s output are collected and output by an optical combiner. Where an optical signal is provided simultaneously at the inputs of two fibers of unequal length, the optical signal in the shorter fiber appears at the combiner at a point earlier in time than the optical signal in the longer fiber. Accordingly, delay may be controlled among the multiple fibers by adjusting the relative differences in length among the fibers.
Longer fibers can be packaged adjacent to shorter fibers by winding at least of segment of the added differential length of the longer fiber onto a spool. Although the spool helps to lessen the space required, it introduces several difficulties. Variations in the physical length and tension of the optical fiber that result as it is wound onto the spool can cause variations in the effective optical length of the fiber. In addition, optical length may vary with movement of the fiber on the spool.
Such variations may be adjusted, for example, by incorporating a heater in the center of the spool to establish a stable thermal profile for the fiber on the spool. However, variations in the geometry of the spool and in the positioning of the fiber on the spool can make it difficult to establish stability in the thermal profile.
Accordingly, it would be desirable to have a multi-fiber optical delay line that is easily connected to associated components, conveniently and compactly packaged, thermally stable and capable of being assembled and operable for high-speed, multi-fiber applications.
A multi-fiber optical delay line incorporates a plurality of optical fibers nestedly positioned so that the individual ones of the plurality of optical fibers are collectively positioned in a single plane. Input ends for each of the plurality of optical fibers are connected to an optical splitter, which thereby provides a copy of an input signal to each of the plurality of optical fibers substantially simultaneously. Each of the plurality of optical fibers has a unique, predetermined length, causing each copy of the input signal that travels over an associated one of the plurality of fibers to arrive at an output end of the associated fiber at the end of a unique interval in time. The nestedly planar structure and geometry of the delay line allows the optical length of each of the plurality of optical fibers to be precisely controlled.
In a first embodiment of the present invention, each of the plurality of optical fibers includes an input segment and an output segment joined by a u-shaped connecting segment. The input segments are substantially parallel, equal in length and proximately positioned so that input ends of the input segments all lie substantially near a straight line that is perpendicular to the input segments. Similarly, the output segments are substantially parallel, equal in length and proximately positioned so that output ends of the output segments all lie substantially near a straight line substantially perpendicular to the output segments. Each of the u-shaped connecting segments of the plurality of optical fibers has a unique, predetermined length. To maintain a planar positioning, each of the plurality of optical fibers is affixed to a planar surface of a substrate.
In a second embodiment of the present invention, the output ends of the plurality of optical fibers are each further coupled to one of a plurality of input ports in an optical combiner, such that the optical combiner superimposes received signal copies from the plurality of optical fibers to produce a single optical signal output.
In a preferred embodiment of the invention, a planar heater is attached to a second planar surface of the substrate so that a temperature profile may be established and stabilized in the plurality of optical fibers.